1. Field of the Invention
The present invention relates to a resist material for use in lithography, for example, in the production of integrated circuits and more particularly to a resist material for two layer resists, which material reproduces negative patterns with high accuracy and has high resistance to oxygen plasma etching (high O.sub.2 RIE resistance). Also, the present invention relates to a method of producing such a negative working resist material, as well as to a process of forming resist patterns with such a negative working resist material.
2. DESCRIPTION OF THE PRIOR ART
Positive type photoresists composed of novolak resin and naphthoquinonediazide as a photosensitizer are featured by having high sensitivity, high resolution, and high solubility in alkaline solutions and have recently been used widely in the field of lithography. Resists are generally grouped into two types, one single layer resists and the other two layer resists (including silylated ones) having a top layer and a bottom layer. While the single layer resists are advantageous that they require simple processes more attention is paid to the two layer resists which can be formed into thin films in order to compensate for decrease in optical focal distance accompanying exposure with high resolution using a short wavelength radiation. In other words, the two layer resists can make the best of the performance of the exposure device used although the processes associated therewith are somewhat complicated.
In the conventional two layer resist system (B. J. Lin, Solid State Technol., 26 (5), 105 (1983)), patterns with a high aspect ratio can be obtained by anisotropically etching the substrate by oxygen plasma etching (O.sub.2 RIE) using a thin film of the resist formed on the substrate as a mask. Hence, much importance is given to the resistance of the resist to O.sub.2 RIE. Generally, those materials which form oxides upon O.sub.2 RIE, typically those containing silicon (Si), are considered to have high resistance to O.sub.2 RIE. In this respect, photoresists currently put into practice are defective in that they have poor resistances to O.sub.2 RIE because they contain no silicon component. In order to avoid the defects, resist materials based on polysiloxane have heretofore been proposed. This type of resist materials generally have low glass transition temperatures. Because of this, there arise various severe problems in adaptability to the processes. More particularly, dusts tend to collect on the material during the processing, the thickness of the film prepared from the material is difficult to be controlled, and the developability of the material decreases due to the deformation of patterns upon development.
For example, conventional resist materials contain novolak resin, a silicon compound and an acid generator, and the silicon compound is rendered alkali-insoluble as a result of condensation with an acid generated by the acid generator upon exposure to high energy radiation (cf., D. R. McKean, et al., "A highly etch resistant, negative resist for Deep-UV and electron beam lithography", SPIE Vol. 1262 Advances in Resist Technology and Processing VII, pp. 110 (1990); and T. Ueno, et al., "Chemical Amplification Negative Resist Systems Composed of Novolac, Silanols, and Acid Generators", SPIE Vol. 1262, Advances in Resist Technology and Processing VII, pp.26 (1990)). The resist materials have low Si contents as low as 12% or less resulting in that they have low oxygen plasma resistance and pattern formation with high precision is difficult therewith.
Another conventional resist material composed of a polysiloxane and an acid generator (crosslinking agent) crosslinks with OH groups upon exposure, resulting in decreased solubility in aqueous alkali solutions (cf. The 51st Scientific Lecture Meeting of Japanese Applied Physics Association, Preliminary Print No. 28a-PC-8 (1990)). Thus, the utilization of crosslinking reaction fails to give acceptable resolution of patterns.
Few resist materials can be used for a wide variety of high energy radiations. Particularly, few are usable for both UV and deep UV which greatly differ in energy level one from another.
Therefore, there is a keen demand for alkali-developable type resist materials which have high glass transition temperatures and high resistance to O.sub.2 RIE and can be used for various high energy radiations.